Thank you very much for your fast advice.
I have tried to disable all plugins (in fact, there are only 2 as this is the initial installation, Askimet and Hello Dolly and they are already inactive). I'm already on theme Twenty Fifteen.
What do you mean by renaming the .htaccess file? To what name?
Anyway, I asked my nephew to come to see me with his laptop. He still has Windows 8 (in case the error is due for difference between Windows 8 and 10), we will try a fresh installation and note down each step with timeline and, if it still doesn't work, we have obviously made a mistake somewhere on the way and will send you a log.
Thanks again for your help, I was just hoping that this is a common problem and that somebody else already had it and now we know how to fix it.
Thanks again.